An effluent treatment system designed for maximum pollutant removal will use a staged injection system in which treatment fluids are injected at several different points in the path of the combustion effluent. This is disclosed in commonly owned U.S. patent applications Ser. No. 022,716 to Epperly, Peter-Hoblyn, Shulof and Sullivan, entitled "Multi-Stage Process for Reducing the Concentration of Pollutants in an Effluent," now issued as U.S. Pat. No. 4,777,024, and Ser. No. 050,198, filed May 14, 1987 in the name of Epperly, O'Leary and Sullivan and titled "Process for Nitrogen Oxides Reduction and Minimization of the Production of Other Pollutants," now issued as U.S. Pat. No. 4,780,289, the disclosures of which are hereby incorporated by reference. Each stage will have different treatment fluid requirements as the pollutant concentration and effluent temperature varies as the effluent progresses through the combustion chamber and past several heat exchangers. Each stage of a treatment system requires a treatment fluid and treatment fluid injector appropriate for the treatment fluid flow rate and the conditions of the combustion chamber.
A particular problem encountered in designing injector systems for certain locations is providing a uniform distribution of treatment fluid across the path of the effluent. In a typical retrofit application, an injector should be able to generate droplets in a selected size range and distribute them uniformly across the boiler cross-section.
Uniform distribution of treatment fluid is most difficult to achieve where relatively low flow rates of treatment fluid are being injected, for example, in the third or fourth stage of a treatment system, where there may be fewer pollutants in the effluent stream. In this situation, it is necessary to contact a sufficient amount of treatment fluid with all of the effluent to reduce the pollutants, yet not to saturate even a portion of the gas stream with chemicals which could increase the pollutant level instead of reacting with the pollutants to reduce emissions. For example, a urea treatment fluid if injected in excessive quantities can cause excessive ammonia in the effluent stream. In prior art injectors such as a pin jet injector used to provide high penetration by a small jet of chemicals at low treatment fluid flow rates there can be a poor dispersion of the treatment fluid across the effluent path and particularly near the effluent passage wall from which the injector extends.
The problems facing the successful method and apparatus for injecting such compositions into an effluent are many. For instance, the heat of the effluent can readily cause a loss in structural integrity of most nozzles or their supports. When the composition to be injected is a solution, often precipitated solute will collect at the end of the nozzle and can plug the nozzle. A plugged nozzle on a treatment fluid injector is not merely an inconvenience: such plugging can render a treatment system ineffective, such that the combustion system does not comply with environmental regulations, necessitating the shut down of production facilities and the loss of time and money. Furthermore, the precipitated solute can break off as chunks and damage the interior of the boiler. Variability of droplet size, degree of dispersion and depth of penetration must be provided for by an injector, depending on the boiler configuration or boiler load. These problems have not been successfully addressed by the prior art.
The prior art relating to injection apparati and nozzles shows that such injection apparati are usually designed for a specific fluid and environment of use. Typically, the prior art teaches mixing of a liquid with an atomizing fluid at or near a nozzle tip, with the liquid being injected into the atomizing fluid at a point concentric within the atomizing fluid conduit. This external mixing of a liquid and atomization fluid can reduce clogging problems. However, this type of design does not allow for deep penetration of a broad band of droplet sizes into a chamber at low liquid flow rates. For example, U.S. Pat. No. 1,625,098, to Rudolph, (an atomizing cleaner) U.S. Pat. No. 3,876,150 to Dwyer (a paint spray nozzle) and U.S. Pat. No. 1,965,465 to Magowan (a liquid fuel burner) show this type of injector layout.
An internal mix injector nozzle, in which a liquid contacts an atomizing fluid before exiting from the nozzle tip, can give a finer control over penetration at low liquid flow rates. However, commercially available internal mix injector nozzles are extremely prone to rapid clogging in a combustion chamber application due to the precipitation of treatment chemicals in the nozzle.
The prior art thus does not teach an injector apparatus suitable for heavy duty use as in a combustion chamber and effluent passage, and which is useful for uniform delivery at low treatment fluid rates in a process for treating pollutants in an effluent stream.
There exists a present need, therefore, for an injection apparatus for injecting a treatment fluid, e.g., an aqueous solution of a NO.sub.x reducing composition into an effluent of the combustion of a carbonaceous fuel, which provides both good dispersion and penetration of injection fluid, and which reduces or eliminates plugging problems in an injector.